Nickel-iron alloys, particularly 36% nickle/63% iron alloys, referred to herein as nickle/iron alloys, are used in the manufacture of printed circuit boards. A commonly used 36% nickle-63% iron alloy is available under the trademark Invar.RTM.. Copper-nickle/iron alloy structures, that is structures having at least one layer of nickle/iron alloy disposed along at least one layer of copper, are used for the manufacturing of high-performance structure that require power planes or wires in contact with dielectric containing fluoropolymers. The mechanical properties of nickle/iron alloy provide dimensional stability to the high performance structures when they are subjected to high temperature and/or high pressures during manufacturing processes, such as lamination. Frequently, there is a need to form holes such as through-holes extending through the nickle/iron alloy layer of copper-nickle/iron alloy structure. Nickle/iron alloy is very difficult to drill; due to the hardness of the nickle/iron alloy, the fine drill bits used to drill through-holes snap while drilling the nickle/iron alloy and require frequent replacement.
Accordingly, chemical etchants are employed to etch a hole in the nickle/iron alloy layer of nickle/iron alloy-copper structures. A photoresist is applied to the copper-nickle/iron alloy structure, and a chemical etchant is applied. The chemical etchant etches any areas not protected by a photoresist, to produce a hole having side walls with irregular surfaces. However, conventional etchants also etch copper and thus etch a hole in the copper layer as well as the nickle/iron alloy layer. For many circuit board designs an etched hole in the copper layer presents no problem or may even be desired. However, in certain applications it is desirable that the copper walls of the hole be uniform; such uniform hole walls are achieved by drilling rather than etching. Thus, when using conventional etchants, the etchants must be prevented from etching the copper layer; to do so, the etching of nickle/iron alloy is stopped prematurely leaving a thin layer of the nickle/iron alloy on the copper. The remaining nickle/iron alloy must then be drilled; the drilling still results in breakage of the drill bits.
It would be desirable to have a technique for etching the nickle/iron alloy layer of nickle/iron alloy-copper structures, and that does not involve drilling the nickle/iron alloy.